The goal of the research in these proposals (with Dr. G.I. Hatton) is to learn how basic membrane properties and local circuit interactions combine with synaptic, osmotic and other non-synaptic mechanisms to cause secretion of peptide hormones from the magnocellular neuroendocrine system of the rat hypothalamo-neurohypophysial system. Initially, the specific aims of the experiments in this particular proposal will be to (1) determine whether cells in the paraventricular and supraoptic nuclei (PVN and SON) are electrotonically coupled, and (2) characterize the membrane properties of the different hypothalamic cell types. Electrophysiological experiments will utilize intracellular recording in vitro in hypothalamic slices or explants, and cells will be classified using intracellular staining and immunohistochemistry for identification of vasopressin and oxytocin and possibly other peptides. The presence of electrotonic coupling will be assessed with dye-coupling after intracellular injection of Lucifer Yellow and with antidromic activation of fast depolarizations. Input resistance, rectification, action potential generation, spike afterpotentials, spike broadening, post-excitatory (tetanic) hyperpolarization and burst firing mechanisms will be examined to determine whether the various types of PVN and SON cells have the same or different membrane properties. As time permits, we intend to determine how a few specific putative excitatory and inhibitory transmitters (e.g., acetylcholine and norepinephrine) and peptides influence the electrical characteristics of immunohistochemical identified cells in PVN and SON. In the long term, we hope to confirm and extend these in vitro results with in vivo intracellular recordings. The strategy of intracellular recording and staining followed by immunohistochemical identification of peptidergic cell type can eventually be used in other hypothalamic nuclei. The research proposed here should provide basic information about the electro-physiological mechanisms responsible for activation of peptidergic neuroendocrine cells in the mammalian brain.